Q: can you provide a more real life example for the procedure regarding old/new licenses?
A: Yes, this should make things clearer:
Lets say you purchased, installed and registered a license for 100 migration, so you have 100 migrations available. But you already know that you must migrate 200 accounts - so you need another license for 100 migrations. As your license is already registered on our systems (control this in the license list tab), you can safely terminate the 'old' license, and purchase another one (100 migrations). Simply install this 'new' license as described above, and register it by opening the email migration extension - you will now have 200 available migrations (see also tab license list). You can repeat this procedure as often as needed.